The present invention relates to a method of powder-metallurgical production of objects, specifically tubes, rods, or the like, in accordance with introduction of a gaseous mixture of nitrogen and hydrogen with a metal powder into a sealed capsule for cold-isotatic pressure prior to hot working.
The objects to be manufactured should consist particularly of stainless steel or high-alloyed nickel steel with good high-temperature characteristics, such as steels containing some 80% of nickel and 20% of chromium. The objects to be produced may, however, also consist of high-speed steels or any other metals and/or metal alloys. The capsules for use in production of these objects are made of a thin metal sheet having a gauge in the range of 1 to 2 mm. For a leakage test of the capsules it has been proposed to introduce helium into the capsule (see German Patent 31 15 095). When this test proved a sufficient sealinig the capsule was charged with a powder of metal and/or metal alloys, then closed and exposed to a cold-isostatic pressure for subsequent hot-working. This type of leakage test, however, has shown that one part of the introduced helium remains in the capsule, in spite of previous flushing with nitrogen or a similar gas, specifically when the capsules are comparatively long and tubular and have a narrow annular gap at one end. Moreover, the diffusion of the enclosed helium gas out of the closed capsule requires a very long time. With a major helium volume inside the capsule the manufactured product tends to become brittle, which is obviously due to occluded helium. Apart from that, the aforementioned method does not allow for leakage testing of the charged and closed capsule, which results in the consequence that the closed capsule may well still present a certain degree of porosity, in particular in the area of the closed powder-charging opening. Thus there is the risk of water penetrating into the capsule during the cold-isostatic pressing step, which leads to actual explosion of the capsule in the subsequent heating step.
The present invention is based on the problem of eliminating the safety risk mentioned last, and, in particular, of obtaining top quality products (with increased homogeneity, improved strength and uniform surface quality, as well as lack of brittleness).